NH2OTHP (0.04 g, 0.37 mmol) was added to a solution of 24 (0.10 g, 0.31 mmol), PyBOP (0.17 g, 0.33 mmol), triethylamine (0.1 ml, 0.74 mmol) in DMF (1 mL). The reaction mixture was stirred at room temperature for 1 h before it was quenched with water, followed by extraction with EtOAc (15 mL×3). The combined organic layer was dried over anhydrous MgSO4 and concentrated under reduced pressure. The residue was purified by silica gel chromatography (CH2Cl2:CH3OH=30:1:1% NH3(aq)) to give a white solid, which was treated with TFA (0.70 ml, 9.44 mmol) in the presence of CH3OH (15 mL) and stirred overnight at room temperature. The reaction mixture was concentrated under reduced pressure to give a white residue, which was recrystallized by CH3OH to afford Compound 24 (0.08 g). 1H NMR (500 MHz, CD3OD): δ 6.45 (d, J=15.8 Hz, 1H), 7.45 (d, J=8.4 Hz, 1H), 7.50-7.65 (m, 5H), 7.81 (d, J=8.4 Hz, 1H), 7.98-8.00 (m, 2H), 8.04 (s, 1H). MS (EI) m/z: 342. HRMS (EI) for C17H14N2O4S (M+): calcd, 342.0674. found, 342.0674.
